jsut use the deletevehicle command, but some peeps has had some problems with it deleted dead units...
deletevehicle <object>
or... if that dont work try this:
but first, create a gamelogic far far away from where all the action is.
<object> setpos getpos <gamelogic>
the line above does not accually delete the object, but it moves it so the player cant see therefore removing lag.